The present invention relates generally to trimming of analogue filters in integrated circuits. More particularly, the invention relates to a method for automatically altering a magnitude of at least one component value in an analogue filter and an automatic adjusting circuit for calibrating an analogue filter in an integrated circuit. The invention also relates to a computer program and a computer readable medium.
The manufacturing process for integrated circuits generally causes a degree of uncertainty with respect to the component values of specific component types. Integrated passive components, such as capacitors and resistors demonstrate undesirable variations in component values, so-called process variations. The value of an actual RC-product in a filter may deviate as much as 30-40% from a nominal value as a consequence of the process variations. Various attempts have already been made to compensate for these detrimental effects.
For instance, the patent document JP, 11274895 discloses a signal processing circuit that is capable of making up for variations of integrated resistors and capacitors by means of adjustable digital filters. A filter coefficient switching means sets the filter coefficient values in a set of digital filters from a pre-defined coefficient table. The filter coefficient switching means chooses such coefficient values that any variation in a signal processing circuit being due to the variation of a semiconductor manufacturing process is compensated for.
The U.S. Pat. No. 5,179,727 describes an automatic adjusting circuit for an analogue filter on a semiconductor chip. The adjusting circuit controls the filter""s parameters such that its centre frequency becomes equal to a reference frequency. The automatic adjusting circuit includes a first phase detector and calibrating filter for coarse frequency tuning and a second phase detector and calibrating filter for fine frequency tuning. The first phase detector produces a signal based on a phase difference between the reference signal and the reference signal filtered through the first calibrating filter, having a low selectivity, and the second phase detector produces a signal based on a phase difference between the reference signal and the reference signal filtered through the second calibrating filter, having a high selectivity. A composite signal is then formed by combining the output signals from both the phase detectors. A DC component of the composite signal is, on one hand, fed back as a control signal to the calibrating filters. On the other hand, the DC component controls the centre frequency of the analogue filter to be controlled to a predetermined ratio with respect to the reference frequency signal by automatically adjusting the centre frequency of the calibrating filters to be equal to the reference frequency signal.
The solution according to the former reference involves digital filtering in series with the analogue filter. Digital filters, however, always cause power losses and introduce a degree of distortion into the signal path. Digital filters are therefore undesired if they can be avoided.
The latter reference, conversely, tunes itself by an analogue manner to a desired centre frequency. This is, of course, a flexible solution that allows a designer to utilise one and the same filter for a multitude of applications in which different filtering characteristics may be demanded. However, the solution involves active filters that per se are relatively noisy and non-linear. This in turn causes distortion and deteriorates the filter performance, which of course, is adverse.
Consequently, the prior art presents various means to either directly compensate for process variations of integrated component values or to alter an analogue filter""s filtering characteristics and thus indirectly compensate for any process variations. However, the proposed solutions are associated with various unwanted side effects, such as power loss, distortion, noise or combinations thereof.
It is therefore an object of the present invention to alleviate the problems above and thus provide an improved solution for handling process variations of integrated component values.
According to one aspect of the invention the object is achieved by a method for automatically altering a magnitude of at least one component value in an analogue filter as initially described, which is characterised by the integrated circuit comprising an adjustable phase shifter for receiving, a periodic reference signal. Based on this reference signal the adjustable phase shifter produces a periodic phase shifted signal. The method involves adjusting a magnitude of at least one component value in the adjustable phase shifter in response to a control signal, such that the phase shift between the periodic reference signal and the periodic phase shifted signal attains a calibrated value, which is as close as possible to a desired value, for instance 90xc2x0. The control signal is in turn generated on basis of a test signal that is produced by a phase detector, which receives the periodic reference signal and the phase shifted periodic reference signal. The method finally involves setting at least one component value in the analogue filter in accordance with a setting of the at least one component value in the adjustable phase shifter, which produces the calibrated value.
According to another aspect of the invention these objects are achieved by a computer program directly loadable into the internal memory of a digital computer, comprising software for controlling the method described in the above paragraph when said program is run on a computer, for instance, a digital signal processor (DSP).
According to yet another aspect of the invention these objects are achieved by a computer readable medium, having a program recorded thereon, where the program is to make a computer, such as a DSP, perform the method described in the penultimate paragraph above.
According to an additional aspect of the invention the object is achieved by an automatic adjusting circuit for calibrating an analogue filter in an integrated circuit. The automatic adjusting circuit includes an adjustable phase shifter that receives a periodic reference signal, and on basis thereof, produces a periodic phase shifted signal. The automatic adjusting circuit also includes a phase detector for receiving the periodic reference signal and the phase shifted periodic signal, and producing a test signal in response to a phase difference between the periodic reference signal and the periodic phase shifted signal. Moreover, the automatic adjusting circuit includes means for producing a control signal on basis of the test signal. The control signal influences a magnitude of at least one component value in the adjustable phase shifter and it is allocated such value that the phase shift between the periodic reference signal and the periodic phase shifted signal attains a calibrated value being as close as possible to a desired value. Finally, the automatic adjusting circuit comprises means for setting the at least one component value in the adjustable filter in accordance with a setting of the at least one component value in the adjustable phase shifter, which produces the calibrated value.
The invention thereby provides an efficient solution, which makes it possible to handle unavoidable process variations of integrated component values.
The invention also offers a competent possibility for continuous compensation for any variations in component values due to temperature variations.
Moreover, the proposed solution includes standardised and relatively uncomplicated building blocks. The invention therefore constitutes an attractive alternative also from a cost and a robustness point-of-view.